The Deadly Wave — Tsunami
from the book, "Our Fascinating Earth"
The Deadly Wave — Tsunami
Named by the Japanese, tsunami refers to the large waves observed in harbors. They are also called seismic sea waves and, less accurately, tidal waves.
These destructive sea waves are generated by sudden movements of the sea floor resulting from earthquakes and volcanic eruptions. Compared with wind—driven waves, which seldom exceed sixty miles per hour, seismic waves may reach speeds of over 500 miles per hour!
Tsunamis travel across the ocean as concentric rings of troughs and crests, like ripples made when a pebble is dropped into a pond, but on a gigantic scale. The height and speed of the tsunami in open sea is determined by the depth of the water. In the deep ocean the length of the tsunami — the distance from crest to crest — may be dozens of miles, while the crest itself may be only two or three feet high. Because the wave is so low, ships at sea often pass over a tsunami without the crew's being aware of it.
When the deadly wave approaches land, the shallow water causes friction and slows up the front portion of the wave to as little as thirty miles per hour. The water behind it then surges forward with concentrated energy. Thus the front of the wave builds up to tremendous heights, at times exceeding one hundred feet, and strikes the land with devastating force.
The highest recorded crest to strike land appeared off Valdez in southwestern Alaska on March 28, 1964, following the incredible Prince William earthquake, which reached a magnitude of 8.5 on the Richter scale. The wave generated by this quake crested at the unbelievable height of 220 feet.
When a tsunami moves into a funnel—shaped bay, its energy is concentrated and it grows to prodigious heights. Such deadly waves are not characterized by the spectacular breakers of storm waves, but instead they arrive as solid walls of water. They strike the land in a series of about eight waves. The first wave is often preceded by a sudden ebbing of the sea from the shore.
A particularly destructive wave hit the northeast coast of Japan on June 15, 1896. This tsunami was generated by a severe submarine earthquake with an epicenter about one hundred miles from the coast. At the head of a bay the waves rose to the awesome height of over one hundred feet. Devastating waves of this size swept an extensive area of Japan's northeast coast. It couldn't have happened at a more inappropriate time, because the beaches were crowded with people celebrating a Shinto festival. More than 27,000 people were killed, and well over 10,000 homes were swept away along a coastal area of over 150 miles.
Japanese fishermen returning home after a long day at sea were shocked and bewildered to find waters near the shore dotted with floating human corpses. At seas the tsunami had crested only one foot high, and the seamen were completely unaware of the violent waves that had passed under their boats. The same waves, unremarkable at sea, destroyed their families and homes when they struck the shore a short time later.
A most expert personal account of a tsunami was given by a scientist from Scripps Institution of Oceanography. He and his wife were staying at Kawela Bay on northern Oahu, Hawaii, when a tsunami struck on April 1, 1946. As the scientist left his house, he saw the water retreat suddenly for such a distance that the coral reefs were exposed. Numerous fish were left stranded on what seconds before had been the floor of the sea.
In a few minutes he could see the water beyond the exposed reef build higher and higher and higher until it suddenly shot forward with amazing velocity. He and his wife took refuge behind the house, which withstood the deluge, and were startled to see neighboring houses reduced to kindling.
As the wave subsided, the couple raced to higher ground before the next, much larger wave hit. There were several more waves, but of smaller magnitude. A total of 173 people lost their lives. Many of the casualties were Hawaiians who went out to the exposed reefs to pick up stranded fish and were killed by the sudden onslaught of the second incoming wave.
The tsunami of 1946 was the last one to take Hawaii by surprise. It originated with a violent earthquake off the northern slope of the Aleutian Trench in the north Pacific Ocean. Five hours later, unheralded, the deadly waves struck Hawaii. Considering the amount of time between the earthquake in Alaska and the arrival of the tsunami in Hawaii, the island people could have been prepared if they had been forewarned.
As a result the tsunami of 1946 became the most important one in history, for it made the people aware that they need endure no more such destruction because of lack of warning. Action was taken that day by the U.S. Coast and Geodetic Survey, and work was begun on a Seismic Sea Wave Warning System.
Since then the islanders have been alerted to coming waves by a most efficient warning system. This was, however, not the end of deaths from Hawaiian tsunami. The fatalities that occurred on May 23, 1960, in Hawaii were due to a reckless curiosity on the part of the victims.
An earthquake off Chile had triggered the tsunami series, which raced across the Pacific at an average speed of 442 miles per hour. The first great wave was scheduled to reach Hawaii in fourteen hours and fifty—six minutes, certainly ample warning to permit an effective evacuation. And so it did, with local people being moved to safety in the highlands. The tsunami was only one minute late!
Still sixty—one people died needlessly. A few of the evacuees tired of waiting in the hills and, thinking the danger was highly exaggerated, returned to their homes in the deserted villages. Most of the sixty—one people who died, however, had remained behind to "see the excitement." They lined the beaches and piers with cameras ready, and sure enough, they were treated to a firsthand view. Accounts that were related by the very few survivors labeled this one an incredible act of foolhardiness.